System and method for processing a work piece by a focussed electron beam

Abstract

A welder utilizes a focussed electron beam which is deflected along the processing path required to weld two pieces together and is also oscillated about the processing path in accordance with a modulation curve. The modulation curve is approximated by a sequence of points. The coordinates of the points in a predetermined coordinate system are read out sequentially from a digital storage and converted to analog signals. The analog signals are subjected to low pass filtering and then applied to the deflection system for the electron beam. The cut-off frequency of the low pass filter is a predetermined multiple of the readout frequency of the digital data. When the velocity of the beam along the modulation curve changes, the cut-off frequency and the readout frequency are adjusted simultaneously. Additionally, the position of the focus and/or the intensity of the electron beam can be varied as the beam traces out the modulation curve. The point-by-point approximation of the modulation curve allows a wide latitude of choice in determining the shape of the curve and the velocity of the electron beam.

We claim: 
     
       1. In a method of processing a work piece by means of an electron beam impinging on said work piece along a processing path having a predetermined direction of movement, said electron beam further being deflected in accordance with a superimposed modulation curve relative to said processing path, said modulation curve having a front surface and a rear surface, the improvement comprising the steps of moving said electron beam step by step along a sequence of points together constituting said modulation curve, and   imparting to said modulation curve mirror symmetry relative to a tangent of said processing path, so that said modulation curve defines a first modulation curve section having a minimal deflection along a direction transverse to the tangent of said processing path, and a second modulation curve section deflected along said transverse direction more strongly than said minimal deflection, and wherein said second modulation curve section has a velocity component in said front surface pointing backwards as seen in a direction opposite to the direction of movement of said electron beam.   
     
     
       2. A method as set forth in claim 1, further comprising the step of momentarily halting said electron beam at each of said points. 
     
     
       3. A method as set forth in claim 1, wherein the path of said electron beam between any two sequential ones of said points constitutes an interpolation path; and wherein said electron beam is continuously deflected along each of said interpolation paths.   
     
     
       4. A method as set forth in claim 1, wherein the distance between sequential ones of said points is a variable distance. 
     
     
       5. A method as set forth in claim 1, wherein said electron beam is deflected by analog signals; wherein said analog signals are derived from digital signals signifying the coordinates of said points; and   wherein digital signals are read out from a digital storage.   
     
     
       6. A method as set forth in claim 5, wherein each of said analog signals has an amplitude corresponding to one of said coordinates; further comprising the step of filtering said analog signals prior to deflecting said electron beam therewith.   
     
     
       7. A method as set forth in claim 6, wherein said filtering is a low pass filtering having a cut-off frequency varying as a function of the readout frequency of said digital storage. 
     
     
       8. A method as set forth in claim 7, wherein said cut-off frequency is a predetermined multiple of said readout frequency. 
     
     
       9. A method as set forth in claim 8, wherein said cut-off frequency is 100 times said readout frequency. 
     
     
       10. A method as set forth in claim 5, wherein said modulation curve has coordinates in a modulation coordinate system; and wherein said modulation coordinate system is rotated while said beam travels along said processing path.   
     
     
       11. A method as set forth in claim 10, wherein said rotation of said modulation coordinate system takes place in dependence on the direction of the tangent to said processing path. 
     
     
       12. A method as set forth in claim 5, wherein the focus of said electron beam is varied while said electron beam travels along said modulation curve. 
     
     
       13. A method as set forth in claim 12, further comprising the step of reading digital control data signifying the position of said focus associated with each of said points from said digital storage; converting said digital control data to corresponding analog control signals;   filtering said analog control signals; and   controlling the position of said focus of said electron beam in accordance therewith.   
     
     
       14. A method as set forth in claim 5, wherein the intensity of said electron beam is varied while said electron beam passes along said modulation curve. 
     
     
       15. A method as set forth in claim 14, further comprising the step of reading out digital intensity data from said digital storage; converting said digital intensity data to corresponding analog intensity signals;   filtering said analog intensity signals; and   controlling the intensity of said electron beam in accordance with the so-filtered analog intensity signals.   
     
     
       16. A method as set forth in claim 5, wherein the frequency of said readout from said digital storage is in the 10 kHz to 10 MHz range. 
     
     
       17. A method as set forth in claim 16, wherein said readout frequency is in the 100 kHz to 1 MHz range. 
     
     
       18. A method as set forth in claim 1, wherein a melt zone having a vapor capillary internally thereto is created in the region of impingement of said electron beam; wherein said melt zone has a front and rear surface as seen in the direction opposite the direction of movement of said electron beam;   wherein the motion of said electron beam relative to the point of impingement creates a flow within said melt zone from said front surface to said rear surface around said vapor capillary; and   wherein said electron beam is deflected by a deflection force having a component corresponding to the direction of flow in the adjacent regions of said melt zone at least in parts of said modulation curve deflected more strongly from said processing path.   
     
     
       19. A method as set forth in claim 18, wherein movement of said electron beam in a direction opposite said direction of flow in said melt zone takes place in said portions having said minimal deflection.   
     
     
       20. A method as set forth in claim 19, wherein said electron beam is continuously deflected along said modulation curve. 
     
     
       21. A method as set forth in claim 19, wherein the velocity of said electron beam along said modulation curve is of the same order of magnitude as the velocity of said flow, at least in said more strongly deflected parts of said modulation curve. 
     
     
       22. A method as set forth in claim 21, wherein the more strongly deflected curve section of said modulation curve has maximum deflection portions relative to said processing path; and wherein the velocity of said electron beam along said modulation curve is between one and ten times the velocity of said flow in said maximum deflection portions.   
     
     
       23. A method as set forth in claim 22, wherein there is a relative velocity between said work piece and the point of impingement of said electron beam; and wherein the velocity of said electron beam along said modulation curve in said maximum deflection portions is between one and twenty times said relative velocity.   
     
     
       24. A method as set forth in claim 18, further including respective connecting sections for connecting said first to said second modulation curve sections; and wherein said electron beam moves along said first modulation curve section in said front surface with a velocity having a component pointed forwards as seen in the direction opposite the direction of movement of said electron beam.   
     
     
       25. A method as set forth in claim 24, wherein said electron beam passes along said second modulation curve section in said rear surface with a directional component pointing backwards, and in said first modulation curve section in said rear surface with a directional component pointing forward. 
     
     
       26. A method as set forth in claim 24, wherein said electron beam passes along said first modulation curve section in said rear surface with a directional component pointing backwards, and along said second modulation curve section in said rear surface with a directional component pointing forwards. 
     
     
       27. A method as set forth in claim 24, wherein said electron beam is at maximum deflection when deflected along said connecting sections. 
     
     
       28. A method as set forth in claim 27, wherein lines of symmetry are formed by tangents to said processing path; and wherein said connecting sections fall outside of said line of symmetry.   
     
     
       29. A method as set forth in claim 27, wherein said connecting sections in said front and rear surfaces are connected by necked-down portions relative to said processing path. 
     
     
       30. A method as set forth in claim 1, wherein the diameter of said modulation curve is between one and five times the width at half intensity of said electron beam at the point of impingement.